OK, as you know, Shiro keeps resetting the war, if the outcome isn't like what he desired. And if Shinji is the biggest obstacle in his war (and who eventually wins at the end), there are probably arguments of why Shiro didn't try to get rid of Shinji ASAP in any timeline. Probably 13 Riders is meant to be made as an answer to that. It's probably made to refute that argument, 13 Riders may be (possibly one of) Shiro's attempt to try to get rid of Shinji. And it still fails. In fact, in the beginning of it, Shinji is being attacked by a monster, there's already a Ryuki (Sakakibara) before him, and he still manages to become a Rider despite all of that, It's just seems that despite his best efforts, Shirou is still unable to eliminate Shinji regardless of what happens.

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Originally Posted by Kamen Rider Die

I was going to watch that alongside Zi-O, since I think that's what it was produced to support?

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Originally Posted by Kurona

Yeah, Rider Time: Ryuki was a Zi-O spinoff and even has Sougo and Geiz in it.

Geiz is Ren, a̶n̶d̶ ̶d̶e̶s̶p̶i̶t̶e̶ ̶t̶h̶e̶ ̶g̶e̶n̶d̶e̶r̶ ̶d̶i̶f̶f̶e̶r̶e̶n̶c̶e̶ ̶S̶o̶u̶g̶o̶ ̶i̶s̶ ̶Y̶u̶i̶.̶ Obviously it's a spin-off and it supports Zi-O, but some people think it's a real continuation of Ryuki for some reason.

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Originally Posted by Kamen Rider Die

Yeah, I'd basically make Present-Day Shiro a cameo appearance, a boogeyman, and have Flashback Shiro as a more slow-collapse villain-in-the-making. The end of the show kind-of portrays his actions as a tragedy, so I think getting to know him before he's an emotionally-distant Mirror Ghost could only help the storytelling.

Shiro's a Woobie Destroyer Of Worlds type (and no, examples of this character type doesn't need to be sympathetic to the audience). I'm into his scientist flashbacks too (or anything involving Shiro's scientific colleagues), but speaking about the show end... looking at the other side, would the tragedy actually a deserving one to him where the finale hinted that things won't go the way he planned them, that his wish will always be out of his reach, despite all the power and invincibility he has and eventually he's not better than any of his victims (he's no exception of the cycle of suffering the Rider War causes)?

Shiro's a Woobie Destroyer Of Worlds type (and no, examples of this character type doesn't need to be sympathetic to the audience). I'm into his scientist flashbacks too (or anything involving Shiro's scientific colleagues), but speaking about the show end... looking at the other side, would the tragedy actually a deserving one to him where the finale hinted that things won't go the way he planned them, that his wish will always be out of his reach, despite all the power and invincibility he has and eventually he's not better than any of his victims (he's no exception of the cycle of suffering the Rider War causes)?

Sure? Maybe? A lot of this stuff is incredibly subjective, and can only be answered by individual viewers. Like, "is one ending better than another" is something I can only answer for myself, and try and explain why. I feel like I've already gone way further down the track of trying to fix Ryuki than I'd ever really want to. It's not my thing, you know? It's a fun thought experiment, but these are all just little feelings I have about what might've been interesting to see play out. They probably wouldn't work as a season-long story with a finite number of episodes and a tight budget!